Cryptosporidiosis is caused by a protozoa coccidia of the suborder Eimeriorina, family Crypotospordidae, genus cryptosporidium. Cryptosporidial species differ from conventionally recognized coccidia (such as Eimeria and isosporia spp.) in at least four ways: (1) Crypotosporidium spp. are much smaller (4 to 6 .mu.m in diameter); (2) although they are thought to invade the epithelia cells of the microvillous border of the intestinal and respiratory tract, they do not invade deep into the tissues like coccidia; (3) in contrast to coccidiosis, there currently are no drugs available for treating cryptosporidiosis; and (4) coccidia are considered host specific, whereas Cryptosporidium spp. are not considered to be host specific.
Although Cryptosporidium has been reported sporadically in the literature for about the past eighty years, only within about the past several decades has it become recognized as a cause of entercolitus in various animals and humans. The cryptosporidia that commonly parasitize the intestinal tracts of calves and humans have been shown to readily infect calves, humans, lambs, piglets and newborn mice. The host range probably includes most newborn mammals. The potential for spread between species, particularly among newborn animals and to humans is well documented.
Intestinal cryptosporidiosis is transmitted directly by the fecal-oral route. Infected individuals shed oocysts in their feces. In contrast to other coccidia, cryptosporidia do not require either an intermediate host or an incubation period outside the host, to sporulate and become infectious. Cryptosporidia are infectious at the time they are shed in feces. One milliliter of diarrheal feces from an infected calf may contain more than 10,000 cryptosporidia oocysts. Thus, the infection can be transmitted readily from calf to calf and even to a human worker via the contaminated feces. In food source animals, such as calves and lambs, diarrhea in the second week of life is the main clinical sign of cryptosporidiosis. In the absence of complicating factors, dehydration remains minimal although emaciation may be evident, appetite may or may not be affected, and the diarrhea subsequently abates within about 8-10 days. In addition to the loose stool from the diarrhea, a characteristic is an offensive rancid-like odor of the stool. Stools are frequently of a wet putty-like consistency, creamy yellow to white in color, and occasionally have small flecks of blood. These stools may vary depending on diet and/or the presence of other infectious agents, which typically result in a very watery yellow liquid stool which readily separates into solid and liquid portions. Uncomplicated cryptosporidial infections of calves do not usually result in mortality. Severe diarrhea with profound dehydration and death in lambs in Great Britain has been reported, which suggests a more virulent isolate in Great Britain in comparison to in the United States in which lambs suffer milder cryptosporidiosis. In any event, there generally are complicating factors in the (animal) host infected with cryptosporidiosis, which factors must be overcome. Supportive care may be necessary, warming in winter, extra fluids above the normal (milk) ration, etc. may be needed.
Currently there are no known efficacious drugs available for treating or preventing cryptosporidiosis in man or animals. In animals, the infection usually occurs in the neonate causing an enteropathy resulting in diarrhea, weight loss, emaciation and sometimes death. Frequently animal-meat producers use various products for treating diarrhea caused by Cryptosporida spp. without beneficial effect, sometimes causing more harm than good.
Attempts to find an efficacious product for cryptosporidiosis frequently looked at treating the animal after the infection had occurred or to determine if compounds would prevent infection. Many compounds have been tried and found not to be effective in eliminating Cryptospodidia infection.